Automatic Repeat Request (ARQ) is a re-transmission protocol in which the receiver checks for errors in the received data and if an error is detected then the receiver discard the data and requests a re-transmission from the sender.
Hybrid ARQ (HARQ) is re-transmission protocol in which the receiver checks for errors in the received data and if an error is detected then the receiver buffers and requests a re-transmission from the sender. A HARQ receiver then combine the buffered data with the re-transmitted data prior to channel decoding and error detection. Hence, even if the re-transmitted data is partially received in error, after combining with buffered data may be error free and no further re-transmission required.
Parallel HARQ processes used to avoid impact on throughput. When one HARQ process waits for the ACK, the second HARQ process can transmit in subsequent TTI. These parallel processes are known as Stop and Wait processes because they stop and wait for ACK from the receiver before sending any further data.
3GPP allows maximum 8 parallel HARQ processes to be configured for an individual UE.
HARQ in HSDPA:
Hybrid ARQ (HARQ) is re-transmission protocol in which the receiver checks for errors in the received data and if an error is detected then the receiver buffers and requests a re-transmission from the sender. A HARQ receiver then combine the buffered data with the re-transmitted data prior to channel decoding and error detection. Hence, even if the re-transmitted data is partially received in error, after combining with buffered data may be error free and no further re-transmission required.
Parallel HARQ processes used to avoid impact on throughput. When one HARQ process waits for the ACK, the second HARQ process can transmit in subsequent TTI. These parallel processes are known as Stop and Wait processes because they stop and wait for ACK from the receiver before sending any further data.
3GPP allows maximum 8 parallel HARQ processes to be configured for an individual UE.
HARQ in HSDPA:
- The number of parallel HARQ processes at any point is dynamic and changes according to rate TTI are scheduled.
- It can use maximum of 8 processes.
- The HS-SCCH is used to inform the UE that which of the HARQ process is being used at any point of time.
HARQ in HSUPA:
- The number of parallel HARQ processes are fixed.
- A HSUPA connection using a 10 ms TTI always uses 4 parallel HARQ processes whereas a connection using 2 ms TTI always use 8 parallel HARQ processes.
- It is not required to inform NodeB that which HARQ process is used at any point of time. The HARQ processes are allocated to consecutive TTI in a cyclic fashion.
This is a simple and straight forward explanation ...helped a lot
ReplyDeleteThanks a lot
ReplyDeleteCan I ask you a question?. You said that "3GPP allows maximum 8 parallel HARQ processes to be configured for an individual UE". Does this mean for Downlink or Uplink?. I am wondering that if eNb has totally just 8 HARQ processes and it is serving many UEs. If just only one HARQ process is active in a TTI and it handles only one packet, it means that only one UE is served in a TTI. I think this is wrong. So maybe, eNb will create a set of 8 HARQ processes for each UE. is this right?
ReplyDeleteThank you
1. HSDPA: number of HARQ processes per UE is a variable, max of 8 processes per UE.
DeleteHSUPA: number of HARQ processes per UE is fixed; 2ms TTI 8 processes per UE, 10ms TTI 4 processes.
2. Each UE has a HARQ entity and each entity contains multiple HARQ processes (e.g. up to 8 in HSDPA). The number of HARQ entities on a Node-B is as many as the number of UE served by the cells, with each having upto 8 HARQ processes.
HARQ was Introduced in which release in UMTS?
ReplyDeleteHARQ was Introduced in which release in UMTS?
ReplyDeleteRelease 5 for HSDPA. Release 6 for HSUPA.
ReplyDeleteNice info. Can you please help sharing UMTS all in one note ? All important short basic notes if you have. Thanks in advance. email : royal.dhuvad@gmail.com
ReplyDeleteDear
ReplyDelete- The 8 processes for 01 UE is at UE side or NodeB side ?
- For less than Average Bit Rate <40, we use Single HARQ or All HARQ ?
- The UE and EnodeB HARQ Entity should utilise the same Number of processes ?